Mobile IP is one of the management methods for mobile Internet systems. Mobile IPv4 corresponding to IPv4 (Internet Protocol Version4) is standardized under RFC3220 (IP Mobility Support). Meanwhile, mobile IPv6 corresponding to IPv6 (Internet Protocol Version6) is now in the process of standardization, Internet draft “draft-ietf-mobileip-ipv6 (Mobility Support in IPv6)” having been created. By those protocols, when a terminal transfers between different networks, the transferring terminal can communicate using the same address.
Furthermore, it has been proposed that, by extending the mobile IP protocol, even where one local network transfers as a unit between different networks, the node belonging to the mobile network is allowed to use the same address for communication. Standardization of the technology for realizing this scheme, known as NEMO, is now proceeding in Internet draft “draft-ietf-nemo-basic-support (Nemo Basic Support Protocol)”.
Specifically, when the mobile network which the mobile router supported by NEMO manages moves away from its home network into a connection with an external network, the mobile router acquires the prefix information of the network to which it is connected currently from among the network information sent from the access router of the network to which the mobile router was connected, to thereby produce a care-of-address to contact the mobile network. Thereafter, the mobile router transmits a binding update message to the home agent. The home agent, after receiving the binding update message from the mobile router, prepares or updates a binding cache entry for associating the home address of the mobile router with a care-of-address.
Then, when a packet from a communication terminal of an external network addressed to a node belonging to the mobile network arrives at the gateway connecting the external network and the home network, the gateway designates the mobile router which has moved away as the next hop router for the node belonging to the mobile network, and relays the packet to the home network. The packet relayed is received by the home agent acting as a proxy, and the home agent encapsulates the packet and transfers it to the care-of-address of the mobile router. The mobile router decapsulates the packet relayed by the home agent and takes out the packet addressed to the node belonging to the mobile network and relayed by the gateway. Then, the mobile router forwards the extracted packet to the mobile network controlled by the mobile router. Due to this, the node belonging to the mobile network can receive the packet.
However, if the gateway fails to receive the routing protocol from the mobile router due to the departure of the mobile network together with its mobile router from the home network and consequently routing table information for relaying a packet to the mobile network is deleted from the gateway, packet transfer from the gateway to the mobile router becomes impossible. This results in the disappearance of a packet addressed to a node of the network which moved.
Methods for preventing such disappearance of packets are given in Document 1 (Mobile Router Tunneling Protocol “draft-kniveton-mobrtr-03.txt”) and Document 2 (Issues in Designing Mobile IPv6 Network Mobility with the MR-HA Bi-directional Tunnel (MRHA) “draft-petrescu-nemo-mrha-02.txt”. In those conventional methods, the home agent receives a binding update and transmits the binding acknowledgement message response to the mobile router. The home agent then receives a routing protocol sent from the mobile router via a tunnel. Thereafter, the routing information received by means of the routing protocol is given to the gateway at the home network.
Document 1 describes a method in which the mobile router gives route information concerning a mobile network under its control to the home agent by use of a dynamic routing protocol. This allows the home agent to act as a router having a path to the mobile network by way of the mobile router. Meanwhile, the home agent updates the routing table depending upon the route information of from the moved mobile router. Furthermore, the home agent notifies the home network of the route information based on an updated routing table whereby the gateway which has been notified transmits to the home agent the packet addressed to the mobile network.
Document 2 describes a method in which the home agent transparently relays from the mobile router to the gateway information on the route to the mobile network, without the use of a dynamic routing protocol. Due to this, the mobile router and the mobile network under its control are detected from the gateway as if they existed in the home network. Meanwhile, the home agent does not update the routing table. Instead, it captures a packet in which the mobile router is designated as the next hop to the node belonging to the mobile network and transmits it to the node belonging to the mobile network by way of a tunnel.
However, in the conventional mobile communication method described in Document 1, a delay possibly occurs in the delivery of routing information from the home agent to the gateway after the home agent receives a binding update message, during which there is a fear of losing the packet addressed to the node of the mobile network. Further, because reliability is low on the communication route from the mobile network to the home agent and an arrival-confirmation message is not in the routing protocol, the routing information from the disengaged mobile router is not assured of arrival at the home agent. Due to this, in the event the routing information from the initial mobile router is not conveyed to the home agent, the information on the route to the mobile network at the gateway is not updated until the next routing protocol is transmitted (usually 30 seconds), and the packets addressed to the node of the mobile network are lost.
Furthermore, in the conventional mobile communication method described in Document 2, in the gateway, the packet address (next hop) to the mobile network is always the address of the mobile network when it was connected to the home network, and also the possibility of losing routing information on the low-reliable communication route due to the routing protocol, so that the home agent is required to capture the packet forwarded from the gateway. This causes high overhead.